This invention relates in general to pressure controls and in particular to a pressure control system for aircraft cabins.
An important aspect of the quality of air in an aircraft cabin is the concentration of oxygen available for the passengers and flight crew. When oxygen concentration in human tissue is deficient, hypoxia occurs. The physiological response of hypoxia varies with the subject, but its effects typically include drowsiness, lassitude, mental fatigue, headache, occasional nausea and euphoria. More serious symptoms include diminished mental proficiency, diminished consciousness, and poor night vision.
Studies have shown that the onset of hypoxia symptoms can occur at various altitudes, becoming noticeably aggravated at altitudes above 8000 feet of pure air. In a typical pressure control system for an aircraft cabin, cabin pressure is measured and controlled to a minimum level of 75,263 Pa, which is the average pressure of pure standard air at an altitude of 8000 feet. However, air within an aircraft cabin is not pure air. The air contains contaminants and moisture. As moisture and contaminants are added to the air, oxygen molecules are displaced. Consequently, for a given total barometric pressure, the partial pressure of oxygen is reduced. Accordingly, the partial pressure of oxygen may be substantially below what is nominally expected at an altitude of 8000 feet. Because existing cabin pressure control systems control total pressure in the aircraft cabin without regard to the partial pressure of oxygen, hypoxia can occur at altitudes below 8000 feet.
An objective of this invention is to provide a cabin pressure control system which controls pressure by reference to the partial pressure of oxygen. Other advantages provided by the invention may become clear in light of the following description, which includes the appended claims and accompanying drawing.